1. Technical Field
The invention relates to a projecting zoom lens installed into a projection display device and the projection display device and, more particularly, relates to a projecting zoom lens for use in a projector system into which a light valve such as a transmission type liquid crystal, a reflection type liquid crystal or DMD (Digital Micromirror Device) is installed and a projection display device.
2. Description of the Related Art
Nowadays the demand for a projection type projector is rising rapidly with the spread of personal computers.
Known as a light valve used in such a projection type projector to optically modulate illumination light based on a video signal, are a transmission or reflection liquid crystal display device, a DMD device in which micromirrors are aligned regularly, and the like.
In order to make the color synthesis of luminous fluxes from the respective light valves or in order to separate the illumination light and the projection light in the reflection type light valve such as DMD, a long back focus is required in the optical system using a plurality of light valves. Furthermore, the following demands are rising.    1. Demand for a high-resolution zoom lens in response to an improvement of a high definition of the light valve,    2. Demand for a zoom lens having a large zoom ratio owing to an extension of the installable area of the projection display device,    3. Demand for a so-called lens shift projecting function of shifting a projection image largely with respect to the projection display device, and    4. Demand for an increase in wide angle, which makes it possible for a viewer to watch a large image at a projection position close to the projection display device
In the case where a higher definition of the projecting zoom lens, a larger zoom ratio, and a wider angle obtained by increasing a diameter of an image circle on the reduction side are achieved in response to the demands 1 to 4, all measures to solve these problems lead to an increase in size of the lens and cause inevitably an increase in cost. Therefore, the aspheric lens is used as a means for achieving such high-performance lens not to cause an increase in size of the lens.
As a zoom lens in which the aspheric lens is used to avoid an increase in size of the lens, known are zoom lenses disclosed in JP 2001-311872 A (corresponding to U.S. Pat. No. 6,587,279), JP 2005-156963 A and JP 2005-266103 A (corresponding to US 2005/0200967 A).
In this case, normally the reduction side of the zoom lens used in the projection display device is constructed as the telecentric system, which constitutes a configurative difference from a zoom lens in an imaging lens.
However, when it is attempted to reduce a size of the lens in which the reduction side is constructed as the telecentric system by any one of technologies disclosed in the above listed publications, a curvature of an S image surface occurs and it is difficult to attain a high definition. A main cause of the curvature of the S image surface depends largely on a negative power of the lens located near a pupil. In the above listed publications, the curvature of the S image surface is corrected insufficiently.
Also, in JP 2001-311872 A (corresponding to U.S. Pat. No. 6,587,279) and JP 2005-156963 A propose a small zoom ratio type of compact zoom lens. However, the back focus of the lens is short and the angle of view is narrow.
Further, JP 2005-266103 A (corresponding to US 2005/0200967 A) proposes a compact zoom lens whose zoom ratio is large and whose angle of view is wide. However, a plastics material is supposed as a material of the aspheric lens in this zoom lens. This plastics material is effective for a cost reduction but is weak at a temperature change. In JP 2005-266103 A (corresponding to US 2005/0200967 A), plastics aspheric lenses are classified into lenses having a positive power and a negative power, respectively, and power ratios among those lenses are specified. However, actually a temperature distribution in the zoom lens is not always kept constant, so that an absolute value of the power itself assigned to a single body of the plastics aspheric lens must be suppressed small. As a result, in the proposed embodiment, the power assigned to a single body itself of the plastics aspheric lens is suppressed small, and therefore an effect of the aspheric lens is also reduced. Also, a temperature rise is easily caused near the pupil in the zoom lens by the influence of reduction in a diameter of a luminous flux, and such condition is unsuitable for the use of the plastics aspheric lens.